Pigmented coating compositions are used in a wide variety of applications including, for example, corrosion resistant primers and decorative topcoats in the automotive, aerospace, industrial and appliance markets. The preparation of pigmented coating compositions generally involves mixing binder resin(s), crosslinker(s), additives, e.g., flow additives and solvents with a compatible pigment dispersion. Pigment dispersions are typically prepared by mixing dry pigment with a pigment dispersant in the presence of a carrier medium, e.g., an organic carrier medium or water. Paint compositions typically include colorant particles dispersed in a resinous binder. The paint composition may further include reflective pigments such as aluminum flake or mica or other color effect pigment compositions or substrate-hiding materials such as titanium dioxide or zinc oxide or lead oxide. The pigment particles used in conventional paints are typically on the order of one micron in size. Particles of this size absorb light at certain wavelengths and scatter light at other wavelengths. This partial absorption and partial scattering creates a degraded coloration effect when viewed by an observer.
Dry pigments are available commercially in the form of agglomerated pigment particles. Pigment agglomerates are more likely to settle out of pigment dispersions and/or pigmented coating compositions, and are accordingly undesirable. To break the pigment agglomerates down into smaller agglomerates and/or individual particles generally requires the use of energy intensive mixing means (commonly referred to as grinding), e.g., sand mills and ball mills. During the grinding process, the pigment agglomerates are broken down into smaller agglomerates and/or individual particles, the surfaces of which are wetted by the pigment dispersant. The pigment dispersant suspends or disperses the pigment particles in the carrier medium and prevents their re-agglomeration on storage. It is desirable that the pigment dispersion remain substantially stable, e.g., showing minimal pigment settling and viscosity change with time, prior to its use in the preparation of a pigmented coating composition.
Organic and inorganic pigments generally are hydrophobic. Thus, pigment dispersants used in pigment dispersions are desirably compatible with both the carrier medium (which may be hydrophilic) and the hydrophobic surfaces of the pigment particles. Such a combination of disparate properties can be achieved with a pigment dispersant having distinct hydrophobic and hydrophilic polymeric segments, i.e., having well defined polymer chain architecture. A wide variety of radically polymerizable monomers, such as methacrylate and acrylate monomers, are commercially available and can provide a wide range of properties including, for example, hydrophilic and hydrophobic properties.
The use of conventional, i.e., non-living or free, radical polymerization methods to synthesize pigment dispersants provides little control over molecular weight, molecular weight distribution and, in particular, polymer chain structure. The continued development of pigment dispersions that are stable and suitable for the preparation of aqueous pigmented coating compositions is desirable. In particular, it would be desirable to develop pigment dispersions that comprise pigment dispersants having well defined polymer chain architecture in which distinct polymer segments are present for efficient dispersion of pigment particles, both organic and inorganic.
One process of radical polymerization referred to as atom transfer radical polymerization (ATRP), described as being a living radical polymerization, results in the formation of (co)polymers having predictable molecular weight and molecular weight distribution. The ATRP process is also described as providing highly uniform products having controlled structure (i.e., controllable topology, composition, etc.). U.S. Pat. Nos. 6,365,666 and 6,642,301 describe (co)polymers prepared by ATRP, which are useful in a wide variety of applications including, for example, pigment dispersants.